1. Field of the Invention
The present invention relates to a glass antenna system provided to a vehicle rear window or vehicle side window glass for receiving AM radio and LF (low frequency) radio broadcast waves, particularly of the kind which can attain good reception of AM radio broadcast waves without a pre-amplifier.
2. Description of the Related Art
In recent years, there is an increasing demand for a vehicle glass antenna for receiving AM and FM radio broadcast waves and television (TV) broadcast waves. To this end, antenna conductive strips are installed on a vehicle rear window glass together with defogging heater strips. However, since such an antenna is disposed in a space left above or below an array of defogging heater strips, an area of space permitted for disposition of the antenna is inevitably small, thus causing a problem that it is difficult to make the receiving sensitivity higher. Further, the antenna must be disposed so close to the vehicle body that the broadcast waves received by the antenna may be leaked to a vehicle body or to a ground side and a DC power source by way of a power supply line for heating. To prevent such leakage, a pre-amplifier or the like is disposed between a receiver and a feed point of the antenna. This arrangement however results in an increased cost and furthermore has a possibility of causing cross modulation when used in a strong electric field. For this reason, an effort has been made for seeking after an antenna which can attain a good receiving sensitivity without using a pre-amplifier. As a result of such an effort, there has been proposed an antenna for improving a receiving sensitivity over all of AM broadcast bands as disclosed in Japanese utility model provisional publication No. 2-72010 or Japanese patent provisional publication No. 7-111412.
In the receiver disclosed in Japanese utility model provisional publication No. 2-72010, attention is paid only to AM broadcast band and a matching circuit is provided within the band for canceling a floating capacity portion-to-ground which is present when an antenna side is observed from an input end of a tuner and thereby improving the receiving sensitivity. In the antenna system disclosed in Japanese Patent provisional publication No. 7-111412, resonance points are present both at the in-band and out-band with a view to improving the receiving sensitivity for AM radio broadcast waves. However, neither of the receiver and the antenna system are structured so as to positively make a local minimum point be present in an adjacent low-frequency side out-band region, so interference by the adjacent out-band waves cannot be avoided.
Further, the both are not adapted so that a similar minimum point is present on a shortwave or low-frequency (LF) band side outside the above described broadcast band and thus suffers interference if a disturbing signal (the disturbing signal is also called an image frequency interference signal) is present in the frequency which is higher than a target receive frequency by twice as high as an intermediate frequency (450 KHz), which is a characteristic phenomenon of a superheterodyne receiver. Thus, when a disturbing signal is present in the frequency range from 1420 to 2530 KHz which is the result of addition of 900 KHz to the receive frequency in the range from 520 to 1630 KHz in Japan, the receiver and antenna system disclosed in the above described Japanese Patent provisional publications cannot avoid an interference.